What should I do between now and September to prepare for University.

Starting University this september to study economics and was wondering what I can do to prepare. Any pre-reading etc...

Economics at university level is more about studying model and using research methods,
Whilst I have list of books that I recommend you get for your academic studies, there are some books that you can read for casual reading without affecting your studies.
If you have a basic understanding of economics (e.g. basic models, basic terminology, theoretical thinking), then the casual reading books would be a fun read, albeit not necessarily helpful for your studies.
Depending on what specific course (for a lack of better word) you intend to take with your prereading, I could recommend specific books for specific purposes. Let me know what you would like to do.

Would greatly appreciate if you could give me a list of books I could read. I didn't study economics at A level so I already went through some basic introductory books for econ.

The basic book to get a foundational knowledge on economics would be those by Gregory Mankiw (https://en.wikipedia.org/wiki/Greg_Mankiw#Textbooks) e.g.
The above books would give you the basic understanding of economics, sufficient for you to understand most economic concepts out there. Do also note, these are also typically first year economics textbooks, so you would likely have them as required texts in at least one of your modules. I would check what the units you would be doing and whether they require one of the above textbooks as mandatory text.
If for any reason you don't like Mankiw (why?), another recommend economics textbook would be Economics by David Begg. It does the same job. I personally prefer Mankiw though.
If your maths is a little rusty, I would recommend going through one of the following:

If you want extra books for your academic studies, I would recommend the following textbooks if your uni have them:

In terms of casual reading for economics, the following are fun reads but you should never quote at uni:

Possible books for casual reading that you can borderline quote include:
If you want a general reading lists from universities, see the following:
https://www.economics.ox.ac.uk/undergraduate-reading-suggestions
https://www.econ.cam.ac.uk/ba/PrelimReadingList.pdf
https://www.lse.ac.uk/research/research-for-the-world/economics/recommended-reads-for-understanding-the-economy-today
https://www.balliol.ox.ac.uk/admissions/undergraduate-admissions/economics-and-management-reading-list
https://www.discovereconomics.co.uk/post/discover-economics-reading-list
https://programs.online.american.edu/econ/masters-economics/resources/economics-reading-list-for-aspiring-economists
https://www.ed.ac.uk/files/atoms/files/econpals_reading_list.pdf
https://www.goodreads.com/shelf/show/economics
If you want to watch YouTube channels, see:
https://www.youtube.com/playlist?list=PL8dPuuaLjXtPNZwz5_o_5uirJ8gQXnhEO (good for introduction to economics)
https://www.youtube.com/playlist?list=PLSQl0a2vh4HDERCw_ddanXbsDpFWcpL-S (good for introduction to economics)
https://www.youtube.com/economicsexplained
https://www.youtube.com/channel/UC0p5jTq6Xx_DosDFxVXnWaQ
https://www.youtube.com/user/Wendoverproductions
https://www.youtube.com/@garyseconomics (you might not want to quote him though)
https://www.youtube.com/@visualeconomiken/featured
The standard economics newspapers and magazines are The Economist (it's in the name) and The Financial Times.
If you have interest in specific areas of economics, you might want to point them out. My niche areas are in financial economics, business economics, macroeconomics, behavioural economics (to an extent), and mathematical economics. I am currently spanning out and venturing in developmental economics and economic history. If you want recommendations
See: https://en.wikipedia.org/wiki/Categoryubfields_of_economics
Whilst I disagree with his rankings (see my interests above), he does outline a number of fields: https://www.youtube.com/watch?v=kGnmMgELyLY
https://www.youtube.com/watch?v=HswUOSe7uuI